This invention relates generally to gas turbine engines, and more particularly to apparatus and methods for mounting shrouds made of a low-ductility material in the turbine sections of such engines.
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine (also referred to as a gas generator turbine) includes one or more rotors which extract energy from the primary gas flow. Each rotor comprises an annular array of blades or buckets carried by a rotating disk. The flowpath through the rotor is defined in part by a shroud, which is a stationary structure which circumscribes the tips of the blades or buckets. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and should generally be minimized.
It has been proposed to replace metallic shroud structures with materials having better high-temperature capabilities, such as ceramic matrix composites (CMCs). These materials have unique mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMCs have a coefficient of thermal expansion (“CTE”) in the range of about 1.5-5 microinch/inch/degree F., significantly different from commercial metal alloys used as supports for metallic shrouds. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/degree F.
CMC shrouds must be positively positioned within the engine in order to effectively perform. Some CMC shrouds have been designed with the shroud component attached to an engine case using a metallic clamping element. While effective for mounting and positioning, these designs can require multiple closely spaced bolts. High bending stress can occur in the bolts, which is contrary to best engineering practice for bolt use.
Other CMC shroud mounting designs avoid the use of a bolted clamp, but transmit high loads from surrounding metallic hardware through the box cross-section of the CMC shroud itself. This reduces the reliability of the shroud segment.
Accordingly, there is a need for an apparatus for mounting CMC and other low-ductility turbine structures that minimizes mechanical loads on the shroud.